Kaposi's sarcoma (KS) is one of the most common and lethal sequela of acquired immunodeficiency syndrome (AIDS). Scatter factor (SF), a mesenchymal cell-derived cytokine, also known as hepatocyte growth factor, is a major constituent of HIV infected T lymphocyte conditioned media (HTLV-II CM). We hypothesize that SF plays a central role in the etiology and pathogenesis of AIDS associated KS by 1) inducing phenotypic conversion of EC that have been "initiated" by AIDS associated infectious agents, i.e. the HHV8 virus, 2) by promoting the clonal expansion of phenotypically converted/initiated EC and 3) sustaining the growth of established KS by stimulating tumor cell proliferation and neovascularization.